Detecting surface defects and form errors with sizes in the micron range has become an important issue for contemporary part manufacturers. Contemporary parts often do not have highly polished surfaces and thus are not amenable to interferometric examination using visible light reflected at normal incidence. For such parts, grazing incidence interferometers may be useful measuring tools. These interferometers use light reflected off a surface at grazing angles to measure the surface's properties.
There are several known types of grazing incidence interferometers, as is described, for example, on pp. 76, 259-260 of the book "Optical shop testing" by D. Malacara, Second Edition (John Wiley & Sons, Inc., New York, 1992). The density of fringes generated by a grazing incidence interferometer having an illumination wavelength .lambda. is characterized by an equivalent wavelength .LAMBDA., given by the formula EQU .LAMBDA.=.lambda./cos(.alpha.) (1)
Since the equivalent wavelength .LAMBDA. is larger than .lambda., many surface textures that are too rough for interferometry at the illumination wavelength .lambda. appear smoother when illuminated at grazing angles than when illuminated at normal incidence. It is consequently easier to perform interferometry at grazing incidence angles for these surfaces.
Various methods have been employed for separating and recombining measurement and reference wavefronts used for grazing incidence interferometry. Abramson, in Optik, 30, 56-71 (1969), describes a well-known type of grazing incidence interferometer that employs a large right-angle prism placed nearly in contact with a sample surface. In this case, the hypotenuse of the prism serves both to separate and to recombine the measurement and reference wavefronts.
Birch, in J. Phys. E: Sci. Instru. 6, 1045-1048 (1973), describes a grazing incidence interferometer that relies on diffraction gratings for separating and recombining the wavefront rather than the hypotenuse of a right-angle prism. Birch's interferometer has the advantage that the sample surface need not be placed close to optical components of the interferometer.
Although there are several types of grazing incidence interferometer in the prior art, including those cited above, grazing incidence interferometry has enjoyed only a limited success for on-line, production yield enhancement of precision manufactured surfaces. There is accordingly an unmet need for an accurate, efficient and convenient geometry for grazing incidence interferometry.